In systems using flexible media, specifically the so-called "floppy disc" systems, the media is confined within an envelope so that it can be stored, handled, mailed, and conveniently inserted into and removed from the floppy disc drive. The envelope has openings for different purposes, including a central opening exposing the central aperture of the compliant magnetic disc. The floppy disc drives are configured so that, when the disc is inserted into operative position, a member on one side of it can engage the central part of the disc against a rotatable spindle on the other, clamping the disc between the members and thus rotating the disc with the spindle, so that recording and reproduction can be accomplished.
In the earliest forms of floppy disc drives, as evidenced by Dalziel U.S. Pat. No. 3,678,481, the clamp and drive arrangement used a solid conical element to fit within the central aperture of the floppy disc. The conical element penetrated within a central concavity in the spindle, centering the disc concurrently with clamping, by moving the conical member into an engaged position. This arrangement was generally satisfactory for initial systems, in which relatively wide tracks (e.g. 12 mils) at relatively low track density (e.g. 48 tracks per inch) were utilized. It was, however, discovered that the edges of the media adjacent the central aperture could be bent, buckled and worn by this arrangement. Furthermore, variations in the central aperture dimension could cause substantial variations in the position of the disc relative to the central axis, materially reducing the reliability of the systems, particularly because floppy discs are used in interchangeable fashion.
There has thus been considerable attention directed toward improving the positioning and clamping structures for floppy disc mechanisms, as evidenced by U.S. Pat. Nos. 4,125,883 to Rolph, 4,208,682 to Bryer and 4,171,531 to Grapes et al, and the patents cited therein. As the art has evolved, the positioner cone, generally a plastic element, has been segmented radially into a number of petals or leaves, which are arranged to be somewhat resilient but substantially stiffer than the deformation characteristics of the floppy disc. There is a wide range of different approaches to the problem, as evidenced by the various patents, because the problems are not simple and require a number of compromises. The problems become more complex as the track widths become smaller (e.g. 4 to 6 mils) and the track density is doubled (e.g. 96 to 100 tpi). Under these conditions, expectable variations in the diametral size of the central aperture of the floppy disc, edge deformation on the floppy disc, and variations in the positioner cone itself, tend to introduce eccentricity of the floppy disc about the central axis.
Prior art systems, while they take a great variety of forms, are usually based upon the thoughts that the leaves making up the positioner cone must be stiffer than the floppy disc, and that centering is to be effected concurrently with clamping. For these purposes, the outer dimension of the positioner cone in the region of full clamping engagement is supposed to match precisely the interior aperture of the floppy disc. This approach has not proven to be satisfactory in practice in meeting the requirements imposed by small track width, high track density systems. One major problem is that each of the parts used must be made very precisely, to tolerances that mean that production yields are very low. Devices in accordance with patents such as U.S. Pat. No. 4,125,883, which discusses centering before clamping, encounter substantial problems in practice because they depend too much upon the precise shape of the positioner cone, which shape cannot reliably be maintained because aging, thermal distortion and wear affect the dimensions and shape of the plastic elements.
The state of the art is evidenced by a structure disclosed in U.S. Pat. No. 4,139,876 to Owens in which the positioner cone is retained in a pivotable frame that can be moved downwardly to engage the positioner cone against a spindle. The positioner cone is somewhat non-compliant and its outline is smaller than the disc aperture. The cone, in practice, is segmented into separate stiff petals. As the cone is seated on the spindle, a follower acts to spread the cone, attempting to center the floppy disc concurrently with the clamping. Such a system buffers both from problems of disc alignment and imprecise centering.